Comparison of Drought Indices in a Semi-Arid River Basin of India

Due to the inherent complexity of drought phenomena, difference in hydro-climatic conditions and watershed characteristics, there is a lack of a universal drought index for assessing drought conditions in a particular region. Hence, the performance evaluation of different drought indices is necessary for identifying a suitable drought index. In this study, the performance of five drought indices was assessed for a semi-arid basin located in western India. The data from nine raingauge stations and four climate stations for the period of 25 years (1985-2009) were used. Based on the data availability, five meteorological drought indices were selected, viz., Percent Departure from Normal (PDN), Effective Drought Index (EDI), Standardized Precipitation Index (SPI), Reconnaissance Drought Index (RDI), and Standardized Precipitation Evapotranspiration Index (SPEI). Among these drought indices, EDI is a time step independent drought index and other four multi-time scale indices were defined at 1-, 3-, 6-, 9- and 12-month scales. The scale for the comparison of these drought indices was chosen based on the correlation with EDI and within among the scales of multi-time scale drought indices. The performance of drought indices during the historical drought was evaluated based on the relative frequency of drought index in a particular drought severity class and the response of these indices with multi-monthly rainfalls. In addition, the performance of these drought indices was assessed using the decision criteria such as Robustness, Tractability, Transparency, Sophistication, and Extendability. Analysis of the results indicated that the 9-month scale is appropriate for comparing drought indices in the study area. SPEI-9 showed maximum relative frequency in the ‘severe dry’ class and was found sensitive to 9-monthly rainfall at most of the stations. Further, the results of the performance evaluation criteria revealed that SPEI-9 has the highest total weighted score (136) followed by RDI-9, SPI-9, EDI, and PDN-9. It is concluded that SPEI-9 is the most suitable drought index for monitoring drought conditions in the study area.